Photographic lens of continuously variable focal length

ABSTRACT

A photographic lens of continuously variable focal length (i.e., a zoom lens) consisting of four lens groups designated from front to rear as groups A, B, C, and D. Group B has diverging refractive power; the other three have converging power. An iris diaphragm is located between groups B and C. Group C has a focal length which is numerically greater (disregarding difference in sign) than the focal length of group B by a factor not less than 1.42 and not more than 1.73. Lens group D is formed of two parts D 1  and D 2 , separated from each other by a clear air space. The first part D 1  has negative power; the second part D 2  has positive power. The focal length of the part D 1  is numerically greater (disregarding the difference in sign) than the focal length of group C by a factor not less than 1.16 and not more than 1.42.

BACKGROUND OF THE INVENTION

The invention relates to photographic lenses or objectives ofcontinuously variable focal length, that is, objectives whose focallength may be varied or adjusted to any desired value within the limitsor range for which the objective is designed. Such lenses, sometimescalled zoom lenses, are well known in general. On example of a lens ofthis general type is disclosed in Basista U.S. Pat. No. 4,303,312,granted Dec. 1, 1981. Another example, which has been available on themarket under the trademark "Vario-Sonar," is disclosed in German FederalRepublic patent No. 1,254,477, issued May 9, 1968, and in thecorresponding British Pat. No. 1,167,114, published Oct. 15, 1969.

The "Vario-Sonar" lens as disclosed in these mentioned patents and asavailable on the market has a relative aperture of f:2.8, and a focallength variable within the limits of 10 mm. to 100 mm. The imaginingperformance and the external dimensions of this known lens areexcellent, and its glass weight of 264.8 grams also fully satisfies therequirements. However, it is no longer competitive today because of itsrelative aperture of f:2.8, the current demand being for faster lensesof larger aperture. In these known lenses, the iris diaphragm isarranged in the base lens.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide an opticalsystem of new design which still satisfies the previously satisfiedrequirements in the field of use of the previously known lens, namely,good coverage for a format of 7.5×10.36 mm. (the picture size when using16 mm. film), but which has a significantly larger aperture than theabove-mentioned f:2.8.

The preferred form of lens of the present invention consists of fourlens groups. The first group (A) is of converging refractive power. Thesecond group (B) is of diverging refractive power. The third group (C)is of converging refractive power, and the fourth group (D) is also ofconverging refractive power. The change in the focal length of the lenssystem is effected, while maintaining its back focus constant, bydisplacing or shifting, along the optical axis, the diverging lens groupB and the converging lens group C relative to each other and relative tothe other two (first and last) converging lens groups A and D.

In such a lens, the above mentioned object is achieved in accordancewith the invention by providing an optical system which ischaracterized, as compared with the prior art, by the followingfeatures:

(1) The iris diaphragm is arranged between the two displaceable lensgroups B and C.

(2) The focal length of the third lens group C is larger than that ofthe second group B, and is at least 1.42 times and not more than 1.73times the focal length of the second lens group B.

(3) The fourth lens group D is formed of two parts or sub-groupsseparated by a clear air space, the first part being of negative powderand the second part of positive power.

(4) The first part or sub-group D₁ of the fourth lens group consists ofa compound member which is formed of at least two lenses and which has afocal length which is greater than that of the third lens group C by atleast 1.16 times the focal length and not more than 1.42 times the focallength of the third lens group C.

When the above listed characteristics or features are considered asrules to be followed in designing the lens system, some variations fromthe specific example disclosed below are possible, as mentionedhereafter.

The advantages obtained by the invention include, in particular, thefact that the objective can have a fairly large relative aperture off:1.8, yet with a rather small front lens diameter, and a large range ofextension or focal length variation is obtained from about 10 mm. to 100mm. The diagonal of the picture format (about 12.7 mm.) is greater thanthe shortest focal length (10 mm.) of the lens system, which isequivalent to penetration into the wide-angle range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view, taken along the optical axis, of alens according to a preferred embodiment of the invention; and

FIG. 2 is a graph of the variable air spaces of this lens as a functionof the focal length.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Numerical data for a preferred embodiment of the invention are given inTable 1, and such a lens system is illustrated schematically in FIG. 1,where the adjustable members are shown approximately in the relativepositions they occupy when the objective is adjusted to a focal lengthof 49.855 mm. and is focussed at infinity.

In FIG. 1, the front of the lens is at the left. Light from the objectbeing photographed passes through the lens from left to right, and formsan image at the image plane (not shown) to the right of the rearmostlens element.

Both in the table and in the illustration, the individual lens elementsare numbered from front to rear in a consecutive series of Romannumerals from I to XVIII. The division of the elements into the fourmain groups A, B, C, and D previously mentioned, as well as the twosub-groups D₁ and D₂, is shown in FIG. 1, and will be apparent in Table1 although not specifically stated there.

Also, both in the illustration and in the table, the vertex or axialthicknesses of the respective elements and spaces between elements aredesignated by the letter d with a numerical subscript corresponding tothe particular thickness or space as numbered from front to rear in asingle consecutive series from 1 to 32. Similarly, radii of curvature ofthe respective lens elements are designated by the letter r with anumerical subscript identifying the particular surface as numberedconsecutively from front to rear, from 1 to 33. The distances andthicknesses d and the radii r are given in the table in millimeters. Asis conventional, positive radii represent surfaces convex toward thefront and concave toward the rear, and negative radii represent surfacesconcave toward the front and convex toward the rear. For cementedsurfaces, only a single r number is given, since the rear face of thefront element of a doublet has the same curvature as the front face ofthe rear element, and no d number is assigned for space between the twocemented elements, since there is no such space.

The refractive index, with reference to the d line, of the glass usedfor each element is given in the column headed n_(d). The Abbe number ofeach glass, also with reference to the d line, is given in the columnheaded V_(d).

                  TABLE 1                                                         ______________________________________                                        Lens                Thicknesses                                               element                                                                              Radii        and Spacings n.sub.d                                                                              V.sub.d                               ______________________________________                                        I      r.sub.1 + 421.790                                                                          d.sub.1                                                                             3.0      1.75520                                                                              27.58                                      r.sub.2 + 64.470                                                                           d.sub.2                                                                             1.8                                                 II     r.sub.3 + 69.282                                                                           d.sub.3                                                                             10.5     1.69100                                                                              54.71                                      r.sub.4 - 453.140                                                                          d.sub.4                                                                             0.1                                                 III    r.sub.5 + 64.470                                                                           d.sub.5                                                                             7.0      1.69100                                                                              54.71                                      r.sub.6 + 339.850                                                                          d.sub.6                                                                             (0.5)                                                                         (40.523)                                                                      (53.897)                                            IV     r.sub.7 + 113.824                                                                          d.sub.7                                                                             1.2      1.71300                                                                              53.83                                      r.sub.8 + 17.278                                                                           d.sub.8                                                                             8.0      1.71300                                                                              53.83                               V      r.sub.9 - 48.000                                                                           d.sub.9                                                                             1.0                                                 VI     r.sub.10 + 25.664                                                                          d.sub.10                                                                            3.2      1.75520                                                                              27.58                                      r.sub.11 + 69.282                                                                          d.sub.11                                                                            2.0                                                 VII    r.sub.12 + 37.585                                                                          d.sub.12                                                                            3.0      1.75520                                                                              27.58                                      r.sub.13 + 79.431                                                                          d.sub.13                                                                            (55.797)                                                                      (15.774)                                                                      (2.400)                                             BL     r.sub.14     ∝                                                                      iris diaphragm                                                                 d.sub.14                                                                           (15.245)                                                                      (4.749)                                                                       (4.829)                                             VIII   r.sub.15 + 1122.000                                                                        d.sub.15                                                                            2.0      1.805518                                                                             25.43                               IX     r.sub.16 + 32.545                                                                          d.sub.16                                                                            8.0      1.61272                                                                              58.63                                      r.sub.17 - 57.460                                                                          d.sub.17                                                                            0.1                                                 X      r.sub.18 + 99.283                                                                          d.sub.18                                                                            4.2      1.71300                                                                              53.83                                      r.sub.19 - 172.776                                                                         d.sub.19                                                                            0.1                                                 XI     r.sub.20 + 127.721                                                                         d.sub.20                                                                            4.4      1.71300                                                                              53.83                                      r.sub.21 - 68.293                                                                          d.sub.21                                                                            (2.528)                                                                       (13.024)                                                                      (12.944)                                            XII    r.sub.22 - 29.853                                                                          d.sub.22                                                                            1.0      1.71300                                                                              53.83                               XIII   r.sub.23 + 20.834                                                                          d.sub.23                                                                            4.5      1.76180                                                                              26.95                                      r.sub.24 + 227.110                                                                         d.sub.24                                                                            10.7                                                XIV    r.sub.25 - 1729.000                                                                        d.sub.25                                                                            3.2      1.69100                                                                              54.71                                      r.sub.26 - 35.737                                                                          d.sub.26                                                                            4.1                                                 XV     r.sub.27 + 112.204                                                                         d.sub.27                                                                            2.2      1.80518                                                                              25.43                                      r.sub.28 + 32.315                                                                          d.sub.28                                                                            2.7                                                 XVI    r.sub.29 + 82.939                                                                          d.sub.29                                                                            3.3      1.46450                                                                              65.77                                      r.sub.30 - 35.228                                                                          d.sub.30                                                                            0.1                                                 XVII   r.sub.31 + 97.166                                                                          d.sub.31                                                                            4.1      1.60311                                                                              60.60                               XVIII  r.sub.32 - 22.386                                                                          d.sub.32                                                                            1.5      1.80518                                                                              25.43                                      r.sub.33 - 92.387                                                      Back focus s' = 28.53                                                         ______________________________________                                    

The first converging lens group A consists of three members (I, II, andIII), of which lens I is a diverging meniscus which is curved toward theimage side. Displacement of the lens group A by 7.243 mm. in thedirection toward the object results in close-up focusing from infinityto 1.5 meters.

The diverging second lens group B consists of at least three members. Ofthese, the member facing the converging third lens group C is aconverging member which is of concave curvature toward the image side.In the preferred form as illustrated in the drawing and specified in thetable, this second group B has four elements, two of which (V and VI)are cemented together.

The converging third lens group C consists of at least three members, atleast one of which has a diverging cemented surface which is curvedconcave toward the image side. As will be seen from the table and thedrawing, this cemented surface, in the exemplary embodiment, is betweenelements VIII and IX.

The refractive power of this third lens group C is so dimensioned, inaccordance with one of the features of the invention, that its focallength is larger in numerical amount (disregarding difference in sign)than that of the second group B, by a factor of not less than 1.42 timesand not more than 1.73 times the focal length of the second lens groupB. Displacement of the lens group C from the wide-angle position by2.736 mm. in the direction toward the object permits an additionalextreme close-up focusing of up to 0.204 meter measured from the filmplane. The image scale, in this focus position, is -0.25, and thedistance of the object from the first lens vertex is 4.6 mm.

The converging fourth lens group D is a multi-lens group and is formedpredominantly of two parts D₁ and D₂. The first part D₁ follows directlyafter the third lens group C and has a negative refractive power. Itconsists of at least two lenses, preferably a compound member. The focallength of this part D₁ is greater in numerical amount (disregardingdifference in sign) by a factor of not less than 1.16 and not more than1.42 times that of the third converging lens group C.

The second part D₂ of the fourth group D has a converging refractivepower and follows the first part D₁ after a clear air space.Displacement of the second part D₂ into this air space can also be usedfor extreme close-up focusing up to an object position directly in frontof the first lens vertex.

A flexible iris diaphragm, schematically shown at BL in FIG. 1, isarranged in the second variable air space, i.e., the space between groupB and group C. The diaphragm preferably maintains the same distance fromthe image plane in all focal length positions. In the case of known zoomlenses, the iris diaphragm is located in the fourth lens group D. Theplacing of the iris diaphragm further toward the front of the lenssystem is one of the important characteristics of the invention. Itpermits smaller front-lens diameters with the adaptation, in accordancewith the invention, of the four lens groups (A to D) to the changedpositions of the pencil of light.

In FIG. 2, the variable air spaces d₆, d₁₃, d₁₄, and d₂₁ are plotted asa function of the focal lengths to which the objective may be adjusted.The focal lengths of the lens system are plotted on the horizontal orabscissa axis, and the lengths of the respective variable air spaces onthe vertical or ordinate axis, the scales along both axes being inmillimeters.

The values given below for the variable air spaces d₆, d₁₃, d₁₄, and d₂₁refer to the different positions of the corresponding members for threedifferent focal length positions of the lens members.

                  TABLE 2                                                         ______________________________________                                                 d.sub.6                                                                             d.sub.13   d.sub.14                                                                              d.sub.21                                    ______________________________________                                        f1 = 10.304                                                                              0.5     55.797     15.245                                                                              2.528                                     f2 = 49.855                                                                              40.523  15.774     4.749 13.024                                    f3 = 97.582                                                                              53.897  2.400      4.829 12.944                                    ______________________________________                                    

It will be noted that in Table 1, these same alternative figures aregiven for the lengths of each of the four air spaces d₆, d₁₃, d₁₄, andd₂₁ which are varied when adjusting the system to a different focallength. In Table 1, as in Table 2, the top figure given for each airspace is the value when the system is adjusted to a focal length of10.304 mm., at or close to the minimum focal length for which the systemis designed; the bottom figure for each space is the value when thesystem is adjusted to a focal length of 97.582 mm., at or close to themaximum focal length for which the system is designed; and the second orintermediate figure for each variable space is the value when the systemis adjusted to a typical intermediate focal length of about 50 mm., i.e.specifically a focal length of 49.855 mm.

The focal lengths of the individual lens groups are: fA (i.e., focallength of group A)=+93.603; fB=-19.527; fC=+30.815; fD=+77.282. Thefocal lengths of the two parts of the fourth lens group D are: fD₁=-39.850; fD₂ =+40.545.

Some variations are possible without departing from the invention. Usingthe specific example data in Table 1 as a guide, and keeping thevariations from the specific data within the limits above stated for thefour characteristic features listed as rules to be followed, a skilledlens designer should be able to design various specific lens systemshaving approximately the same advantages and giving approximately thesame beneficial results as a lens system constructed according to thespecific example. When making changes from the specific data given inTable 1, it is advisable to adapt the manufacturing data to theindicated data to such an extent that none of the surface powers (Δn/r)differs in amount by more than 0.05·1/fA from the values contained ineach case in the data table, fA being the focal length of the convergingfirst lens groups A.

What is claimed is:
 1. A photographic lens of continuously variable focal length consisting of four lens groups comprising, in sequence from front to rear along an optical axis:(a) a first group (A) of converging refractive power; (b) a second group (B) of diverging refractive power; (c) a third group (C) of converging refractive power; and (d) a fourth group (D) of converging refractive power; (e) said lens groups being so dimensioned and arranged that a change in the focal length is effected by displacing diverging lens group B and converging lens group C along the optical axis relative to each other and relative to groups A and D while maintaining a constant back focus;said lens being characterized by the features that: (f) an iris diaphragm (BL) is located between group B and group C; (g) the third lens group C has a focal length which is numerically greater than the focal length of lens group B by a factor not less than 1.42 and not more than 1.73; (h) the fourth less group D is formed of two parts (D₁ and D₂) separated from each other by a clear air space; (i) the first part D₁ of the fourth lens group is of negative power; (j) the second part D₂ of the fourth group is of positive power; (k) said first part D₁ comprises a compound member formed of at least two elements; and (l) the focal length of said first part D₁ is numerically greater than the focal length of the third lens group C by a factor not less than 1.16 and not more than 1.42;the respective elements of the lens being dimensioned substantially in accordance with the data in the following table, the respective symbols in the table having the meanings explained in the accompanying specification:

    ______________________________________                                         Lens               Thicknesses                                                 element                                                                               Radii       and Spacings  n.sub.d                                                                               V.sub.d                                ______________________________________                                                r.sub.1 + 421.790                                                       I                  d.sub.1   3.0       1.75520                                                                               27.58                                   r.sub.2 + 64.470                                                                           d.sub.2   1.8                                                      r.sub.3 + 69.282                                                        II                 d.sub.3   10.5      1.69100                                                                               54.71                                   r.sub.4 - 453.140                                                                          d.sub.4   0.1                                                      r.sub.5 + 64.470                                                        III                d.sub.5   7.0       1.69100                                                                               54.71                                   r.sub.6 + 339.850                                                                                    0.5                                                                  d.sub.6   40.523                                                                         53.897                                                   r.sub.7 + 113.824                                                       IV                 d.sub.7   1.2       1.71300                                                                               53.83                                   r.sub.8 + 17.278                                                        V                  d.sub.8   8.0       1.71300                                                                               53.83                                   r.sub.9 - 48.000                                                                           d.sub.9   1.0                                                      r.sub.10 + 25.664                                                       VI                 d.sub.10  3.2       1.75520                                                                               27.58                                   r.sub. 11 + 69.282                                                                         d.sub.11  2.0                                                      r.sub.12 + 37.585                                                       VII                d.sub.12  3.0       1.75520                                                                               27.58                                   r.sub.13 + 79.431                                                                                    55.797                                                               d.sub.13  15.774                                                                         2.400                                             BL     r.sub.14   ∞                                                                         iris diaphragm                                                                           15.245                                                               d.sub.14  4.749                                                                          4.829                                                    r.sub.15 + 1122.000                                                     VIII               d.sub.15  2.0       1.805518                                                                              25.43                                   r.sub.16 + 32.545                                                       IX                 d.sub.16  8.0       1.61272                                                                               58.63                                   r.sub.17 - 57.460                                                                          d.sub.17  0.1                                                      r.sub.18 + 99.283                                                       X                  d.sub.18  4.2       1.71300                                                                               53.83                                   r.sub.19 - 172.776                                                                         d.sub.19  0.1                                                      r.sub.20 + 127.721                                                      XI                 d.sub.20  4.4       1.71300                                                                               53.83                                   r.sub.21 - 68.293                                                                                    2.528                                                                d.sub.21  13.024                                                                         12.944                                                   r.sub.22 - 29.853                                                       XII                d.sub.22  1.0       1.71300                                                                               53.83                                   r.sub.23 + 20.834                                                       XIII               d.sub.23  4.5       1.76180                                                                               26.95                                   r.sub.24 + 227.110                                                                         d.sub.24  10.7                                                     r.sub.25 - 1729.000                                                     XIV                d.sub.25  3.2       1.69100                                                                               54.71                                   r.sub.26 - 35.737                                                                          d.sub.26  4.1                                                      r.sub.27 + 112.204                                                      XV                 d.sub.27  2.2       1.80518                                                                               25.43                                   r.sub.28 + 32.315                                                                          d.sub.28  2.7                                                      r.sub.29 + 82.939                                                       XVI                d.sub.29  3.3       1.46450                                                                               65.77                                   r.sub.30 - 35.228                                                                          d.sub.30  0.1                                                      r.sub.31 + 97.166                                                       XVII               d.sub.31  4.1       1.60311                                                                               60.60                                   r.sub.32 - 22.386                                                       XVIII              d.sub.32  1.5       1.80518                                                                               25.43                                   r.sub.33 - 92.387                                                       Back focus s' = 28.53                                                          ______________________________________                                    


2. A photographic lens of continuously variable focal length consisting of four lens groups comprising, in sequence from front to rear along an optical axis:(a) a first group (A) of converging refractive power; (b) a second group (B) of diverging refractive power; (c) a third group (C) of converging refractive power; and (d) a fourth group (D) of converging refractive power; (e) said lens group being so dimensioned and arranged that a change in the focal length is effected by displacing diverging lens group B and converging lens group C along the optical axis relative to each other and relative to groups A and D while maintaining a constant back focus;said lens being characterized by the features that: (f) an iris diaphragm (BL) is located between group B and group C; (g) the third lens group C has a focal length which is numerically greater than the focal length of lens group B by a factor not less than 1.42 and not more than 1.73; (h) the fourth lens group D is formed of two parts (D₁ and D₂) separated from each other by a clear air space; (i) the first part D₁ of the fourth lens group is of negative power; (j) the second part D₂ of the fourth lens group is of positive power; (k) said first part D₁ comprises a compound member formed of at least two elements; and (l) the focal length of said first part D₁ is numerically greater than the focal length of the third lens group C by a factor not less than 1.16 and not more than 1.42;the respective elements of the lens being so dimensioned that one of the refractive powers Δn/r of the lens differs by more than 0.05 times 1/fA from the corresponding refractive power of a lens dimensioned in accordance with the data in the following table, fA being the focal length of the converging first lens group A, and the respective symbols in the table having the meanings explained in the accompanying specification:

    ______________________________________                                         Lens               Thicknesses                                                 element                                                                               Radii       and Spacings  n.sub.d                                                                               V.sub.d                                ______________________________________                                                r.sub.1 + 421.790                                                       I                  d.sub.1   3.0       1.75520                                                                               27.58                                   r.sub.2 + 64.470                                                                           d.sub.2   1.8                                                      r.sub.3 + 69.282                                                        II                 d.sub.3   10.5      1.69100                                                                               54.71                                   r.sub.4 - 453.140                                                                          d.sub.4   0.1                                                      r.sub.5 + 64.470                                                        III                d.sub.5   7.0       1.69100                                                                               54.71                                   r.sub.6 + 339.850                                                                                    0.5                                                                  d.sub.6   40.523                                                                         53.897                                                   r.sub.7 + 113.824                                                       IV                 d.sub.7   1.2       1.71300                                                                               53.83                                   r.sub.8 + 17.278                                                        V                  d.sub.8   8.0       1.71300                                                                               53.83                                   r.sub.9 - 48.000                                                                           d.sub.9   1.0                                                      r.sub.10 + 25.664                                                       VI                 d.sub.10  3.2       1.75520                                                                               27.58                                   r.sub. 11 + 69.282                                                                         d.sub.11  2.0                                                      r.sub.12 + 37.585                                                       VII                d.sub.12  3.0       1.75520                                                                               27.58                                   r.sub.13 + 79.431                                                                                    55.797                                                               d.sub.13  15.774                                                                         2.400                                             BL     r.sub.14   ∞                                                                         iris diaphragm                                                                           15.245                                                               d.sub.14  4.749                                                                          4.829                                                    r.sub.15 + 1122.000                                                     VIII               d.sub.15  2.0       1.805518                                                                              25.43                                   r.sub.16 + 32.545                                                       IX                 d.sub.16  8.0       1.61272                                                                               58.63                                   r.sub.17 - 57.460                                                                          d.sub.17  0.1                                                      r.sub.18 + 99.283                                                       X                  d.sub.18  4.2       1.71300                                                                               53.83                                   r.sub.19 - 172.776                                                                         d.sub.19  0.1                                                      r.sub.20 + 127.721                                                      XI                 d.sub.20  4.4       1.71300                                                                               53.83                                   r.sub.21 - 68.293                                                                                    2.528                                                                d.sub.21  13.024                                                                         12.944                                                   r.sub.22 - 29.853                                                       XII                d.sub.22  1.0       1.71300                                                                               53.83                                   r.sub.23 + 20.834                                                       XIII               d.sub.23  4.5       1.76180                                                                               26.95                                   r.sub.24 + 227.110                                                                         d.sub.24  10.7                                                     r.sub.25 - 1729.000                                                     XIV                d.sub.25  3.2       1.69100                                                                               54.71                                   r.sub.26 - 35.737                                                                          d.sub.26  4.1                                                      r.sub.27 + 112.204                                                      XV                 d.sub.27  2.2       1.80518                                                                               25.43                                   r.sub.28 + 32.315                                                                          d.sub.28  2.7                                                      r.sub.29 + 82.939                                                       XVI                d.sub.29  3.3       1.46450                                                                               65.77                                   r.sub.30 - 35.228                                                                          d.sub.30  0.1                                                      r.sub.31 + 97.166                                                       XVII               d.sub.31  4.1       1.60311                                                                               60.60                                   r.sub.32 - 22.386                                                       XVIII              d.sub.32  1.5       1.80518                                                                               25.43                                   r.sub.33 - 92.387                                                       Back focus s' = 28.53                                                          ______________________________________                                     